FIG. 1 is a block diagram showing an example of a conventional exhaust gas re-circulation system using a diaphragm which is employed in the system disclosed, for example, in JP-A-6/147025. In the drawing, reference numeral 1 designates a four-cycle engine for automobiles, powered by the combustion of a gas mixture comprising fuel and air. Numeral 2 denotes an intake pipe line, one end of which is connected to the engine 1 for supplying the gas mixture to the engine 1, and numeral 3 designates an air cleaner connected to the other end of intake line 2 for removing dust contained in the outside air as well as for feeding air to the intake line 2. Numeral 4 shows an injector provided at the middle of the intake line 2 for injecting fuel including gasoline into the intake pipe line, and numeral 5 designates a throttle valve for regulating the amount of the mixed gas to be fed into the engine 1. Further, numeral 6 shows an exhaust pipe line connected to the engine 1 at one end for expelling the gas mixture (exhaust gas) generated by combustion in the engine 1, and numeral 7 denotes a purifying apparatus disposed at the other end of the exhaust line 6 for purifying the exhaust gas with a three way catalyst or the like and for expelling the processed exhaust gas outside. Alternatively, the injector is located at a position designated by numeral 4' when the fuel is injected directly to the combustion chamber or sub-combustion chamber as in the case of a diesel engine.
In addition, numeral 1a shows a combustion chamber; 1b is an intake valve for closing communication between the intake line 2 and the combustion chamber 1a;
1c is an exhaust-gas valve for closing communication between the exhaust pipe line 6 and the combustion chamber 1a; and 1d is a piston which moves vertically in the combustion chamber 1a.
Next, the operation of a four-cycle type gasoline engine is described as an example.
Initially, both the intake valve 1b and the exhaust-gas valve 1c are closed. When the intake valve 1b of the four-cycle engine 1 is opened, the piston 1d moves down to feed air to combustion chamber 1a from the intake line 2 through the cleaner 3. Subsequently the gas mixture mentioned above can be fed into the combustion chamber 1a instead of air by appropriately activating the injector 4. At the same time the amount of the gas mixture actually fed into the combustion chamber 1a can be regulated by controlling the degree of opening the throttle valve 5. The intake valve 1b is then closed, and the piston 1d is driven upward to compress the gas mixture. In this manner, the air and fuel contained in the gas mixture react together to produce a combustion gas of high temperature and high pressure in the combustion chamber 1a. Then the piston 1d is driven downwards by the force of volume expansion due to the combustion of the mixed gas, and the force acting on the piston 1d results in the driving force. In this case, combustion may be forcibly induced by use of an ignition plug or like means. Finally, the exhaust-gas valve 1c is opened in synchronism with the upward movement of piston 1d so that the combustion gas in the combustion chamber 1a is expelled outside through the exhaust pipe line 6 and purifying apparatus 7. Thus, the automobile four-cycle engine 1 can output driving force continuously by repetition of the above operation.
In the case of the four-cycle engine 1 for automobiles, when the exhaust gas is discharged from the exhaust line 6, hazardous components, such as nitrogen oxides (NO.sub.x), contained in the exhaust gas are eliminated by a chemical such as three way catalyst provided in the purifying apparatus 7.
Next, the above exhaust gas re-circulation system is described.
In FIG. 1, reference numeral 8 denotes an exhaust gas re-circulation system for re-circulating exhaust gas to the intake pipe line under certain conditions; 15 is an exhaust gas intake pipe line for sending the exhaust gas from the exhaust-gas line 6 to the exhaust gas re-circulation system 8; and 16 is an exhaust gas re-circulation pipe line for re-circulating the exhaust gas to be returned from the exhaust gas re-circulation system 8 to the intake pipeline. Further in the exhaust gas re-circulation system 8, numeral 9 designates a housing secured to the exhaust gas intake line 15 and exhaust gas re-circulation line 16; 10 is a re-circulation passage provided in the housing 9 for communication of the exhaust gas intake line 15 with the exhaust gas re-circulation line 16; 13 is a valve seat formed in the housing 9; and 11 is a closure valve for closing the re-circulation passage 10 when in abutment with the valve seat 13. Numeral 12 designates a movable shaft to one end of which is secured the closure valve 11 so that when the shaft 12 is moved in a predetermined direction, the valve 11 is in abutment with or detached from the valve seat 13; 14c is a diaphragm fixed to the housing 9 for controlling movement of the movable shaft 12 in a predetermined direction; 14b is a spring for biasing the closure valve 11 in the closing direction; 14a is a diaphragm chamber for introducing negative pressure; and 14d is a check valve for checking the negative pressure.
Next, the operation of re-circulating the exhaust gas is described.
Initially, the closing valve 11 is in abutment with the valve seat 13 to close the re-circulation passage 10. When negative pressure is introduced in the diaphragm chamber 14a, the force of the valve opening direction defined by multiplying the negative pressure by the surface area acts on the diaphragm 14c. If the force is larger than the biasing force of the spring 14b in the valve closing direction, the movable shaft 12 and the closure valve 11 secured to one end thereof displace, whereupon the re-circulation passage 10 communicates with the intake pipe line 2. Thus, the exhaust gas returns into the engine combustion chamber 1a through the intake line 2. Consequently, combustion in the automobile four-cycle engine 1 is suppressed by the amount of non-flammable exhaust gas returned to the combustion chamber 1a.
The suppression of combustion in the automobile four-cycle engine 1 can further inhibit temperature increases in the combustion gas or the engine even in the case of lean-burn type operation where the mixing ratio of fuel to air is low. Accordingly, increased levels of NO.sub.x associated with temperature increases of the combustion gas or of the engine can be also controlled.
However, conventional exhaust gas re-circulation systems as constituted above have the following problems.
First, when the differential pressure between the intake gas and the exhaust gas of a diesel turbo car or similar type is high, it is necessary to increase the biasing force of spring 14b to properly operate the valve against such high differential pressure. Therefore, it is necessary to enlarge the diaphragm 14c as well as the system itself. Secondly, negative pressure must be generated to act on the diaphragm 14c. In general, in gasoline-type engines, the pressure in the intake pipe line between the throttle valve 5 and the automobile four-cycle engine 1 serves as the source of negative pressure. On the other hand, in diesel engines, the pressure in the brake vacuum pump provided for the automobile brake system is used for negative pressure. Therefore, the system can not be operated in gasoline engines where negative pressure is not generated. Moreover, even if operable, it is difficult to minutely regulate the negative pressure. In diesel engines, the problem arises that negative pressure for brake operation must be used for another purpose (i.e., the exhaust gas re-circulation system). Consequently, it is necessary to set the amount of re-circulated exhaust gas to a sufficiently low level so as not to cause knocking or conspicuous loss of power which will be the result of excessive re-circulation of the exhaust gas. A further problem is that NO.sub.x emissions become difficult to reduce as a result.
We have proposed an exhaust gas re-circulation system, for example, in JP-A7/332168 in which a motor is used in place of the diaphragm. FIG. 2 is a cross-section showing an example of such a conventional exhaust gas re-circulation system using a motor. In the drawing, numeral 17 denotes a stepping motor which is fixed to the housing 9 for controlling movement of the movable shaft 12 along a predetermined direction. The stepping motor has an internally threaded structure for converting rotational movement to linear movement so that the movable shaft 12 is moved vertically when the motor is rotated. Other components are substantially the same as in the diaphragm type exhaust gas re-circulation system of FIG. 1, and therefore are not described but only shown by like reference numerals.
According to the system in FIG. 2, the exhaust gas re-circulating operation can be performed, without the aid of negative pressure, by driving of the closure valve 11 and movable shaft 12 using the stepping motor 17. Moreover, it is possible to downsize the exhaust gas re-circulation system by employing a small sized stepping motor.
However, if the use of the stepping motor 17 is associated with considerably high pressure exhaust gas or increased amounts of returned exhaust gases, an enlargement of the closure valve is needed. Lack of thrust force in the motor may lead to the inability to move the closure valve or other problems. In particular, in diesel turbo-type cars, the maximum pressure of the exhaust gas is as high as 2000 mmHg and requires a very large amount of re-circulated gas flow. However, the above system is totally inoperable in such cases.
In diaphragm-type systems, although the system is tightly closed by the check valve 14d to maintain the valve in a preset open state after a desired level of negative pressure has been applied, the pressure of the exhaust gas may be changed by pulsation of the exhaust gas. Therefore, the pressure effecting on the valve is also changed so that the valve slides to change its degree of opening.
The present invention was made to solve the above problems. Therefore, it is an object of the present invention to provide an exhaust gas re-circulation system, in which the closure valve 11 can be easily moved even though a motor is used as a driving mechanism for driving the closure valve 11. Furthermore excellent NO.sub.x emission reduction, superior to that effected by the conventional diaphragm type system, can be obtained even in diesel turbo-type cars or the like vehicles.